Dennis White: He always used to say, "OK. What do you want to talk about?" That was the cool thing. It was whatever you wanted.

Whether picking through the minefield of Dennis' childhood memories, or recent his divorce, Dr. Karpf was there and ready to listen. Ready to write a prescription if that was what Dennis needed to mend his troubled mind.

Dennis White: It felt like someone that I could really count on. Somebody that I could trust, and that he was completely there for me.

Hoda Kotb: Was it actually working for you?

Dennis White: Yes

Hoda Kotb: It was. And how could you tell that, Dennis?

Dennis White: Because I was able to start to function again.

What Dennis White didn't know is that just as he was starting to pull himself together, his psychiatrist, Dr. Richard Karpf, was coming unraveled.

Looking back, Dennis says, there were subtle signs that something was wrong.

Dennis White: What he explained to me, was, is that I could make a lot of money.

Dennis says he was flattered that his doctor, a man with several degrees on his wall, wouldoffer him, a mechanic, a chance to go into business with him. But the more Dennis learned about his psychiatrist’s business plan, the more skeptical he became.

Dennis White: You know, it seemed to me to be like one of those tier, pyramid things, you know.

In the end, Dennis says he invested roughly $3,000 in the venture before he gave up and wrote it off.

Dennis White: This was the first time that he went from, I’m the patient, he's my doctor. And that was the boundary. OK? And I really wasn't comfortable with it, even then. But I did it.

Hoda Kotb: When someone knows all your weak spots, they know where all the Achilles heels are. Did that make you uncomfortable that he, in a sense, had all that power over you?

Dennis White: Yeah, so you start to feel that you owe him something, maybe.

After years of successful therapy, Dennis White knew he owed his psychiatrist a debt of gratitude.

But he couldn't have imagined the extraordinary price he believed his doctor expected him to pay.

After a hectic holiday season, after all the stress and commotion, Dennis White just wanted to sit in the quiet of his therapist's Long Island office and talk.

But the minute he walked into that office, on the day after Christmas 2002, it was obvious to the patientthat his psychiatrist was the one who needed help.

Dennis White: His clothes were a mess. And his shirt was open. And his pants were kind of open.

Hoda Kotb: Did you ask him what was wrong?

Dennis White: That's when it all began. Yeah.

Dennis says seeing his therapist sitting in front of him, disheveled and down in the dumps, he forgot all about his own troubles.

Dennis White: And I was like, "You know, it’s OK. You know, you can talk." I said, "You were always there for me." And he started to open up a little bit. And it was just done in like little increments. He wouldn't just blurt things out.

Hoda Kotb: He said he had some problems, but he wasn't specific?

Dennis White: One of the problems I remember him telling me about was he had a problem with a female patient. He was telling me that he knew that it was, kind of like a wrong thing for him to be doing and he was very worried.

For the rest of that session, Dennis says, the roles of patient and therapist were reversed.

Though vague, Dennis says Dr. Karpf told him the former female patient was out of control--an abuser of drugs and alcohol and sexually aggressive towards him. Dennis says Dr. Karpf said he was worried that she could even cause him to lose his license.

Hoda Kotb: What was that like when the guy who you were telling all of your secrets to was suddenly revealing this, sort of, bombshell secret to you?

Dennis White: I was like "Wow, I can do something for you." Or, you know, I felt I could be helpful.

He had no idea what it would turn into. Dennis says that over the next few days, Dr. Karpf asked him if he could help him get a used van. But, he says, the doctor didn't want it registered in his own name. Dennis also says he wanted help renting a boat, too. But again, the doctor didn't want his name associated with it in any way.

Dennis White: He's like, "Well, I want to, you know, rent a boat. And I just need to find a place where there's shark invested waters."

Hoda Kotb: What did you think?

Dennis White: I didn't know if he just wanted to, you know, maybe hunt the sharks or fish for sharks.

By now, Dennis says, he and his psychiatrist were talking almost daily--not in the doctor's office, but in Dennis's shop.

Dennis White: I was usually running around, or it was busy. But I did feel awkward, you know?

Not only that, but Dennis says their conversations seemed to focus exclusively on Dr. Karpf and that problem he was having with a female patient.

Hoda Kotb: What did he say that made you go, "What?"

Dennis White: Well, he asked me if I would help him take care of a problem that he had. And I said, “Is it the same problem that you were telling me?" He says, "Well, it's relative to that." He says, "but there's other people involved." He said, you know, “I would really need to get my hands on a good gun." And he says, "Would you know anybody"

At first, Dennis thought it might be part of his therapy. A test of his trust and commitment to his psychiatrist.

Then Dr. Karpf added a devastating detail.

Dennis White: He indicated to me that it had to have a silencer. Like the biggest silencer a gun could have.

Dr. Richard Karpf: If anybody asks you anything--we never had this conversation

Dennis White: I know, I know.

In a nerve-wracking turn of events, the police asked Dennis to wear a police wire to record more details of what his psychiatrist was up to.

Dennis White: I was almost at the point where I told them I really am not comfortable. They were like, "well s--, don't worry. You know we'll have a you know, we'll protect you."

At their next meeting, Dr. Karpf was intent on learning more about Dennis' contact, the guy Dennis said could get him a gun.

Dr. Richard Karpf: The whole idea is that if your man is willing to guarantee me some high quality product--you know, in that area--you know, that's not going to fail on me.

Dennis White: As far as the gun failing you mean?

Dr. Richard Karpf: Yeah, right.

With the police now listening to their every word, Dennis tried to talk his psychiatrist out of going through with it.

Dennis White: You're a good doctor. You've got a good practice. I’ve seen your degrees. You worked so hard to get that. OK? I actually look up to you for that. And it troubles me that maybe you're going to throw it away because of what somebody else did.

But Dr. Karpf was not backing off. In fact, he told Dennis that murder--under the right circumstances--can be therapeutically beneficial.

Dr. Karpf: There are a lot of things that are very, very moral, that are very, very emotionally unhealthy. The fact is it may be the right thing to do in order to have the most emotionally healthy situation.

After hearing the tapes, police were convinced Dr. Karpf intended to kill somebody. But they still didn't know who.

The only person on earth who could find out was Dennis White.

Dennis White: I didn't want to make a mistake. I didn't know how far out of his mind he was. And he was coming always at night now, when there was nobody there. I was like always having that thought of "Well, what if he thought he told me too much?”

For Dennis White, New Years Day, 2003, dawned with dark foreboding.

His psychiatrist for the past six years had not only been talking about murder, he'd hooked Dennis into helping him get him a potential murder weapon.

Dr. Richard Karpf: The point is, I’m not asking you to do the job for me. In other words. I'm doing the job.

For days, while wearing a police wire, Dennis White prodded his psychiatrist for more specifics, asking what could possibly have pushed Dr. Karpf to a point where he thought the only possible solution to his problem was murder.

Dennis White: What could have happened that made you feel like that?

Dr. Richard Karpf: They physically hurt me.

Dennis White: OK, like did they beat you up?

Dr. Richard Karpf: Yep. I mean it wasn't bad enough to go into the hospital, but it was pretty humiliating. It's something that I haven't forgotten about and I’m never going to forgive them.

Karpf told Dennis this was not going to be a single killing--what he had in mind was a mass murder.

In vivid detail that fired Dennis' imagination, Karpf said he intended to invite six people to a dinner party in Manhattan and coldly murder them all--one at a time.

Dennis White: While they were having dinner is when he was going to surprise them. Go up there. That's why he needed the silencer, OK--because it was an apartment building, he told me, somewhere in midtown.

Dennis White: He was going to shoot them first. And then chop them up, you know, and I had to get cleaning fluids to clean up the blood.

Hoda Kotb: And then he was going to transport the pieces in garbage bags?

Dennis White: And bring them down the elevator and load them in the van. And then we'll go to the boat. And then, you know, we'll bring them out to sea. We'll dump them. And he said that'll be the end of it.

Hoda Kotb: How did that register with you? The man who was supposed to be rock-solid, helping you, was now talking crazy.

Dennis White: It just felt like an out of body thing.

Missing from Dr. Karpf's lurid scheme were the names of his intended victims.

Convinced that Dr. Karpf was too dangerous to be walking the streets, the police put veteran undercover officer Michael O’Leary on the case to work with Dennis.

Michael O’Leary: I instructed Dennis White to make contact with Karpf and and get me on the phone with Dr. Karpf, so I could personally talk to him.

The next day, Dennis told Karpf he'd made contact with a friend of his who could--for the right price-- get him an untraceable gun with a silencer and all the ammo he would ever need.

Dennis White: He was like, "You sure you know the guy?" He was also, you know, a little leery. And I said, "Yeah, I know him." He says, "All right." He says, "I can't wait to get it."

Michael O’Leary: He told me that he wanted something that he would be able to conceal in his coat pocket. That he wanted--that this was going to be up close and personal.

Dr. Richard Karpf: We're talking up close and point blank range.

Michael O’Leary: OK. Does your guy know you're coming?

Dr. Richard Karpf: Well, ahhhh--of course not.

Karpf also insisted that his new contact provide him with four 12-bullet clips--an astounding amount of ammunition--for the job Karpf said he had in mind.

Michael O’Leary: What the f--- is this with four clips? What are you going to do? Sit there and reload. Do you know how to load one, first off?

Dr. Richard Karpf: I’ve never actually loaded one, no.

O’Leary: I even said to him, "Are you up do this task of doing this?

Michael O’Leary: It seems like you're taking a big step here and you really don't know what you're getting into as far as even having…

Dr. Richard Karpf: Well, it's not going to used tomorrow, let's put it that way.

O’Leary: You know, give me more money, I’ll do it for you. Just let's do that. And he wasn't giving that up. He was giving up that he was going to do it. It seems like a personal thing that he was going to take care of this.

Karpf: But the thing is, somebody fouled me, alright. Do you understand that? Somebody fouled me. And unfortunately the system is kind of failing me.

Dr. Richard Karpf: I don't feel safe doing this all alone. All I want is Dennis.

Michael O’Leary: OK, bring him. You want me to bring him or do you want to bring him?

Dr. Richard Karpf: I’ll bring him.

Michael O’Leary: He didn't appear to be the criminal type. He knew very little about the gun, how it was going to work and how things were going to be. But I got the sense that he was going to follow thru with what he wanted to do.

All Dennis White wanted to do was walk away from the whole sordid mess, but now he was stuck.

Both Dr. Karpf and the police had made certain that Dennis White--a man prone to panic attacks--was the central character in this drama's final act.

In just two weeks time, Dennis White's relationship with his psychiatrist, Dr. Richard Karpf had been turned totally upside down.

Suddenly, he found himself counseling the doctor who'd been treating him for six years and recording their conversations for the police.

Trying to talk him out of what sounded like a plot to settle a string of old scores with a gun, the patient pointed out to the doctor that, for anyone who'd wronged him in the past, success was really the best revenge.

Dennis White: You know how you got even? Because you’ve got a good practice, right. So you could say, "Hah--touché!"

Karpf was charged with four felony counts, ranging from conspiracy to commit murder to illegal gun possession.

Now the psychiatrist who'd made his living probing the minds of others found himself on the couch, trying to explain himself to the state's forensic psychiatrist.

Dr. Richard Karpf: I just fed him that story about the shark invested waters and stuff because I wanted to keep the conversation going.

The story about dinner party executions--dismemberment and sharks--was just talk. According to Dr. Karpf, all of it--including the gun purchase--was really about creating a bonding experience with his patient, Dennis White.

Dr. Karpf: Now remember, I didn't really want the gun that much. I mean the gun wasn't really what was so important. I wanted his friendship, OK, and this whole thing was just an elaborate way of kind of, I’m sorry, involving myself with him.

That's right. According to Dr. Richard Karpf, the whole thing was just his convoluted way of creating a little quality time with Dennis White. Why?

Dr. Karpf: I was feeling very lonely in my life at that particular time and he was very easy to talk to.

To understand how a psychiatrist might think involving a patient in a fantasy murder plot is OK, one has to venture deep into the mind of Dr. Richard Karpf to a place where painful memories so warped his judgment in late 2002 that the line between reality and fantasy simply vanished.

Dr. Karpf: I was very naive about people.

Drawing on a patchwork of painful memories, Karpf told lawyers and psychiatrists his problems began in childhood, when social awkwardness made him a magnet for bullies.

Dr. Richard Karpf: I think a next door neighbor was bullying me and I guess my parents felt I needed to be protected from him. I think that was about it.

Karpf says the pattern of being victimized by others continued into adulthood.

Dr. Karpf: When I was in med school back in the seventies, for example, I, I, uh, I chose a roommate down there just so that I could start. And my roommate took advantage of me, I mean, he found two other friends that were more to his liking and they just locked me out of my apartment and just kept ridiculing me until finally wound up moving out.

It remains a painful, humiliating memory.

But Dr. Karpf also recalls another incident from med school which seems to rank as one of his most cherished memories from that period of his life.

An experience that combined fantasy with just a hint of violence to win him the acceptance and approval of his fellow students.

Karpf says they were all taking a break from classes, wandering through a flea market in Mexico, when he spotted a stiletto on a table.

He later recalled it in a videotaped deposition.

Dr. Karpf: I made up a story about wanting to you know, about wanting to do away with somebody that, that I didn't like or something like that. It was purely made up. And they just, they sort of like pretended to buy into it, you know, and we always had a good laugh over it.

Karpf says he never forgot that afternoon of college camaraderie, or that it was his fantasized tale of revenge that made him the center of attention.

Dr. Richard Karpf: I may have had fantasies about wanting to get even but I never actually acted out on them.

Dr. Karpf says the elements for his revenge fantasy were drawn from movies.

Righteous killers like Dirty Harry provided inspiration.

Hints for disposing of bodies came from the "Jaws" movies.

Lawyer: Where did you learn about silencers?

Dr. Richard Karpf: It's common knowledge. Look, it's been portrayed in movies. It's ahh..there was a movie, I guess, that was made many years ago --Magnum Force with Clint Eastwood. I don't know if you--

Lawyer: Right.

Dr. Richard Karpf: --remember the movie, you know where there, there was a policeman that , that formed a vigilante squad and they were, they were eliminating people using silencers.

Eliminating people.

Could that be what Dr. Karpf had in mind when he told Dennis White his plan for dinner party executions?

Had he actually drawn up a guest list? Or was the dinner party idea merely a faceless fantasy?

Less about flesh and blood victims than a collection of long simmering insults from his past.

Dr Karpf: But I never forgot the experience, see. It was really. It was really so intense, so humiliating that you know, I’m not going let them get away with it. There are some things you just don't let people get away with.

There seemed to be a place at the table for everyone who'd tormented him.

The neighborhood bully. The computer salesman. The alcohol abusing patient. The med school roommate or even--his own sister.

But if it was all a fantasy, why involve Dennis White at all? Why buy the gun, silencer and bullets?

In Dr. Karpf's mind, the answer was simple.

Dr. Karpf: It's very similar to what I did in medical school when I bought the stiletto. I, I bought the stiletto and I bought ahh the gun because I felt that in both cases I simply wanted somebody's recognition, warmth and an presence, OK? Just so I could have it. You know, so I could feel better.

The murder plot was all a big fantasy, according to Dr. Karpf. But investigators weren't so sure.

A search of Karpf’s home turned up chilling clues.

Research on sharks that Dr. Karpf had downloaded off of the internet---and on a "things to do today" memo pad, there were handwritten notes titled "motives for murder."

Michael O’Leary is the undercover officer who sold Karpf the gun, silencer and bullets.

Michael O’Leary: It's hard to say what goes on in somebody's mind. Is he dangerous? Could he be a danger? I believed when I was dealing with him that he was intent on hurting somebody.

But beyond that, the psychiatrist also faced a showdown with his former patient: Dennis White.

Karpf: I’m not going to stand for it anymore. And I’m tired of being humiliated after 50 years.

According to Dr. Richard Karpf, all the talk about a dinner party massacre was nothing more than a fantasy.

The culmination of 50 years of pent-up frustration--and a way to form a friendship with his patient, Dennis White.

Dr. Karpf: I simply wanted to look tough in front of Dennis White. I wanted to look tough and I wanted to show him how, I don't know, how much--I didn't want to tell him in those words "I want to be with you"

If that sounds crazy to you, you are not alone.

Dennis White: Well, if I want to have a friend, maybe, you know, ask him, you know, "want to go to dinner? Want to hangout?" You know, I don't think that asking him to go into a murder plot is, like, really healthy, do you?

Psychiatric evaluations of Karpf determined that the doctor may have had a psychotic episode in late 2002 and that he likely suffered from undiagnosed mental disorders for most of his life.

Dr. Karpf: My whole thinking at the time was totally illogical.

Though Karpf was initially charged with four felony counts, including conspiracy to commit murder, prosecutors eventually decided that it was not clear he intended to actually kill anyone.

So, in a deal with prosecutors, Karpf agreed to plead guilty to a single weapons charge and surrender his medical license in return for a 5-year probation and ongoing psychiatric care.

To Dennis White, that seemed like a slap on the wrist considering the turmoil Dr. Karpf caused him.

Dennis White: I mean, he was the trained person, not me. I was going for the help.

So Dennis was determined to pursue justice on his own terms.

Dennis White: I feel like, violated, you know? Cause he knew to pick me. He knew my weaknesses. And he knew which buttons to push to get me to get as far as we got. Because I know he never expected to get arrested.

In May 2008, five years after Karpf’s arrest, the doctor and his former patient were face to face again. This time, in a Long Island courtroom.

Dennis White had sued for malpractice and was demanding money as compensation for the way, he said, Dr. Karpf had abused their doctor-patient relationship.

Ruth Bernstein: He is still unrepentant and he still doesn't get the damage and wreckage that he's left in his wake.

Ruth Bernstein is Dennis White's lawyer. She also represents two other former patients who are also suing Dr. Karpf for of malpractice.

Ruth Bernstein: They have a greater need of treatment because they're now traumatized on top of whatever problems they originally had, and they are less able to seek treatment and to benefit from it, because they have an impaired ability to form a relationship of trust.

Though Dr. Karpf did not agree to be interviewed, during the trial Dr. Karpf said that, as a doctor, the most he was responsible for was having an inappropriate conversation with a patient.

Karpf: I thought I could confide in him and tell him something racy and he wouldn't use against me in any way.

Bernstein: Did you feel he betrayed your friendship?

Karpf: Yes.

Karpf told jurors he was surprised that Dennis took his tale of murder and mayhem seriously.

Karpf's lawyers admitted that the doctor committed malpractice but they argued Dennis White's award for damages should be minimal because he seemed to be functioning fine.

To prove its point, the defense had a private investigator follow Dennis around, videotaping his every move.

Dennis: At first blush you would say, "gee, they haven't yet invented a camera that can shoot the insides of somebody's brain--so you would think why would anyone every surviell somebody in an emotional injury case?” But I think they were hoping to get him at a party. Which he doesn't go to parties. Or doing something where they could show that he really wasn't affected at all.

In the end, both sides agreed to a monetary settlement which Dennis says will enable him to get the kind of therapy he needs to recover from his experience with Dr. Karpf.

Hoda Kotb: Do you think justice has been done?

Dennis: The only reason why I’m going to say no is because he doesn't have remorse for what he did. And now I went from that trust to fearing him, you know?

To be sure, there is no evidence that Dr. Karpf intended to kill anyone. But Dennis White says one thing is clear: his former psychiatrist has a long memory and if that dinner table still exists in his imagination, there is now a place for him.

Dennis White: The reason why he was doing his first act was because he was humiliated, OK? I mean, I definitely went like a few tiers beyond that, ok what I did to him. I definitely betrayed him. In his mind anyway.

Two other malpractice cases against Dr. Richard Karpf are scheduled for trial in September. Dr. Karpf is defending against both of those lawsuits.

In a highly-anticipated speech to Congress Tuesday, Israeli Prime Minister Benjamin Netanyahu argued that a potential nuclear deal being negotiated by major powers including the United States "paves Iran's path to the bomb."